Field of the Invention
The present invention relates to high density memory arrays based on ReRAM devices, and particularly bipolar operation of ReRAM devices.
Description of Related Art
Resistive random access memory (RRAM or ReRAM) is a type of nonvolatile memory that includes a programmable resistance material, such as transition metal oxide, which can be caused to change resistance between two or more stable resistance ranges by application of electrical pulses at levels suitable for implementation in integrated circuits. The voltage and current settings applied to the ReRAM memory device determine whether the ReRAM memory device undergoes SET operations to establish a lower resistance state, or RESET operations to establish a higher resistance state.
Some ReRAM technologies are configured for “bipolar” operation, in which opposite current directions are used for reducing resistance (e.g. SET) and increasing resistance (e.g. RESET). For reliable and efficient operation of ReRAM devices, it is desirable to use well-controlled current and voltage sources for both current directions.
ReRAM cells can be configured in large arrays with bit lines, source lines and word lines. An example array can be configured so each memory cell in the array has a selection device such as a select transistor, used to connect or disconnect a current path through the memory element of the cell in response to a word line voltage. The example array can include local bit lines coupled to columns of cells, and by a local bit line decoder to global bit lines. The global bit lines can be coupled by column decoders to sense amplifiers and to controlled current/voltage circuits used to apply bias conditions to selected cells. Also, the example array can include local source lines coupled to the cells in a column of cells, and by source line decoders to global source lines or a common source line. The global source lines or common source line can be coupled to controlled current/voltage circuits used to apply bias conditions to selected cells. Select transistors in the memory cells can be connected between the memory element and the source line circuits, or alternatively, between the memory element and the bit line circuits. A large array can include several decoding stages as known in the art.
Current flow in one direction through a selected memory cell can be established using a controlled current/voltage source connected via a bit line, while the source line is set to a reference such as ground. Current flow in the opposite direction changes the roles of the bit lines and source lines so that the current flow can be established using a controlled current/voltage source connected to a source line using source side decoders, while the bit line is set to a reference such as ground.
Current though the memory cells can be controlled to some degree using the controlled current/voltage sources coupled via the decoding stages to the memory cells. However, the loading on the controlled current/voltage sources can be significantly different in the two opposite direction current paths. This makes precise control at the memory element in all the cells in the array very difficult, and particularly so when supporting bipolar operations. Variations in biasing conditions at the memory elements can increase the variation in the timing and accuracy of the programming operations in the array. Reducing these variations can lead to improvements in speed and reliability of memory using ReRAM technology.
It is desirable to provide a new bipolar programming scheme for ReRAMs to accurately control ReRAM devices.